Hydrogels are biocompatible polymers with highly tunable mechanical properties. Synthetic hydrogels are tissue-like in several ways, e.g., being soft, wet, and water-permeable, making them popular biomaterials in tissue engineering applications and biomedical devices. However, bio-tissues generally have a low friction (or high lubricity). For example, it is reported that the coefficient of friction of a bio-tissue, cartilage, is between 0.01 and 0.02 (Caligaris M, Ateshian G. A., Osteoarthritis Cartilage. 2008 October; 16(10):1220-7). In contrast, hydrogels may have relatively high friction (or low lubricity). Such an inadequate lubricity may hinder their wide applications as bio-tissue substitutes. It would be desirable for a hydrogel to have a lubricity that would be equal or superior to the lubricity reported for cartilage.
Lubricity describes the slipperiness of a surface, and generally can be characterized by its friction coefficient or coefficient of friction (CoF) which is measured in vitro as the ratio of the horizontal friction force between two bodies and the force pressing them together (or normal force). The lower the CoF is, the more lubricious the surface. Recent studies indicate a correlation between the in vitro measurements of coefficient of friction (CoF) of hydrogel contact lenses and subjective comfort (Brennan N A., Optom Vis Sci 2009; 86:e-abstract 90957; Coles C M L, Brennan N A., Optom Vis Sci 2012; 88:e-abstract 125603; Kern J, Rappon J, Bauman E, Vaughn B., Invest Ophthalmol Vis Sci 2013; ARVO E-Abstract 494; Jones L, Brennan N A, Gonzalez-Meijome J, Lally J, Maldonado-Codina C, Schmidt T A, Subbaraman L, Young G, Nichols J J, members of the TIWoCLD, Invest Ophthalmol Vis Sci 2013; 54:TFOS37-70; Subbaraman L. N. and Jones L. W., Contact Lens Spectrum 28:28-33 (2013); Fonn D., Contact Lens Spectrum 28:28-33 (2013)).
Unlike most physical properties of a material, CoF is not an intrinsic material property, but instead should be considered more correctly as a system property, because it depends upon many variables of a biomedical device (e.g., a contact lens) under testing and of a testing system, including materials used, a probing substrate against which a contact lens under test is moved, contact mode (e.g., a constant point of contact, a moving point of contact), normal force pressure, moving speed relative to each other, and lubricating fluid between the probing substrate and the testing lens, etc. Different methods has been developed/used in measuring in vitro the lubricity of contact lenses, such as, a tribometer (Rennie A. C., Dickrell P. L., Sawyer W. G., Tribology Letters 2005, 18:499-504; Roba M., Duncan E. G., Hill G. A., Spencer N. D., Tosatti S. G. P., Tribology Letters 2011, 44:387-97; U.S. Pat. No. 6,940,580), atomic force microscopy (Kim S. H., Marmo C., Somorjai G. A., Biomaterials 2001, 22:3285-94; Kim S. H., Opdahl A., Marmo C., Somorjai G. A., Biomaterials 2002, 23:1657-66), an inclined plane method (U.S. Pat. No. 8,480,227), lubricity ratings based on digital rubbing of lenses between the fingers (U.S. Pat. No. 8,480,227). However, results obtained by using those previously reported methods may not be compared to judge the true lubricity, because they all are system properties, depending upon the system used. In addition, they are not suitable for determining the in-vivo lubricity of a contact lens or a biomedical device, because a contact lens or biomedical device must interact with a soft-wet cornea or bio-tissue, not with a hard solid substrate.
Recently, Dunn, Sawyer and Angelini developed a method for determining friction coefficients (CoF) of hydrogel materials in a “Gemini” testing system (Dunn A. C., Sawyer W. G., Angelini T. E., Tribology Letters 54:59-66 (2014)). According to this method, CoF tests are carried out by moving a hydrogel sample against a hydrogel substrate, i.e., using “Gemini” soft wet hydrogel surfaces as interaction surfaces in the testing system. The Gemini testing system is similar to a biological system, e.g., the glycocalyx of the eyelid rubbing against the corneal glycocalyx in the eye. But, the CoF obtained by using this Gemini testing system is not an intrinsic material property, but instead is a system property. It would be desirable to use a non-system property to characterize (or measure) lubricity of a hydrogel.
Therefore, there is still a need for methods for determining and controlling the lubricity of a biomedical device made of a hydrogel and for developing and producing biomedical devices with a targeted lubricity. There is also need for a biomedical device with a target lubricity that is equal or superior to the lubricity reported for cartilage.